1. Field of the Invention
This invention relates to a method for minimizing formation of oxides of nitrogen, while substantially eliminating carbon monoxide, hydrogen and volatile hydrocarbons in the off-gas stream of a molten metal refining vessel. The volume and energy of resultant gases are minimized to facilitate subsequent pollution control processing.
2. Description of Related Art
Increasing environmental concerns and regulations, in combination with melting and refining vessels utilizing operating procedures to increase production, lower operating costs and improving product quality, necessitate providing improved apparatus and methods for controlling environmental pollutants associated with off-gases from such melting and refining vessels.
Carbon monoxide (CO), resulting from chemical reactions within the refining vessel is of major environmental concern. Volatile hydrocarbons (VOC""s) are also of environmental concern especially in operations wherein the furnace charge contains undesirable high levels of plastic, paint, and/or other organic matter. Uncombined free hydrogen (H2) is also of concern, mainly because of its potentially explosive property. Quantities of some of such off-gases are increased when energy for heating and melting is increased by addition of various forms of fossil fuels such as oxygen-fuel gas through burners in the vessel, inclusion of coal and/or petroleum coke with the vessel charge or injection of coal or graphite into the molten bath.
Elimination of such gaseous pollutants, by prior practice pollution reducing means which are operated at elevated temperatures, forms oxides of nitrogen (NOx) which are also of environmental concern and subject to air quality regulation.
Prior practice methods and apparatus for controlling emissions of undesirable gases include combusting CO-containing off-gases from an oxygen steel converter by the induction of ambient air to such off-gases in a hood of the converter, followed by quenching resultant gases removed from the hood, as described in U.S. Pat. No. 3,592,630; contacting off-gases of a basic oxygen furnace with oxygen, in a hood above the opening of the furnace, to oxidize the CO, while also contacting the off-gases with a stream of air or steam to increase the turbulence of the gases, as described in U.S. Pat. No. 4,040,819; and an apparatus for controlled after burning of process exhaust gas wherein fresh air and xe2x80x9cpurified process exhaust gasxe2x80x9d are introduced into the process gas to control temperature of the gas and the concentration of oxidizable substances, described in U.S. Pat. No. 4,983,362. Although, such prior art recite apparatus and methods for reducing CO emissions, a method is not found for minimizing formation of NOx while lowering the volume and energy of exhausted gases.
The present invention provides a system to substantially eliminate CO, H2 and VOC""s from the off-gas stream of a molten metal refining furnace or vessel while minimizing formation of NOx during such elimination process. Practice of the invention significantly minimizes both volume and energy of resultant exhaust gases thereby lowering the volume of diluting ambient air added before baghouse treatment. Such practice decreases total gas flow to the baghouse as compared with prior practice hence reducing total emitted solids.
Molten metal refining furnace and vessel off-gases are directed to a reaction chamber having an injection system for oxidizing and cooling agents, temperature and gas-composition measurement apparatus, and controls for metering injection of agents such as: atomized water and pure oxygen or dry compressed air, which act as mixing agents as well, into the chamber so as to substantially maintain a selected temperature range for oxidation reactions and to control input of pure O2 or compressed air within a predetermined-volume- range.
Reaction temperature of gases within the reaction chamber is controlled within a narrow range to promote substantially complete oxidation of CO, H2 and VOC""s while deterring formation of NOx. Injected quantities of dry compressed air or pure O2 are closely controlled so as to limit the volume of exhausted gases and further suppress chemical formation of NOx, when compressed air is injected as the source of O2. The evolution of the off-gases, change in volume and composition throughout the refining process and the treatment is continuous and changing to meet the demands.
Other specific features and contributions of the invention are described in more detail with reference being made to the accompanying drawing.